1. Field of The Invention
The invention relates to fastening devices for combining two plate-like pieces together, and particularly to the fastening device for tightly attaching a heat sink to a CUP mounted on a daughter board.
2. The Related Art
Traditionally, a CPU in the computer is installed unto a PGA (Pin Grid Array) socket which is mounted on a mother board, and a heat sink is attached to the CPU by a clip which is generally hooked unto the side edges of either the CPU or the socket, for example, U.S. Pat. Nos. 5,381,305 and 5,436,798. Recently, the PGA sockets were replaced by the so-called the slot 1 connector with the associated retention means whereby a daughter board module with the CPU thereon is retained thereto wherein the heat sink is tightly fastened, by four screws, to the case or enclosure of the daughter board module for removal of heat generated by the CPU in the daughter board module.
To lower down the cost of the daughter board module, an attempt is made to remove the enclosure of the daughter board module. Thus, it is required to have the a clip directly hooked unto the daughter board instead of the enclosure. One approach is taken that a plate-like clip with four hook legs on the four corners presses the back surface of the daughter board thus having the CPU on the front surface thereof abut against the heat sink wherein the four hook legs extend through the holes of the daughter board and are respectively latched to holes of the heat sink. Anyhow, it is not easy to make such plate like clip, and moreover, it requires to use a specific tool for simultaneously having the four hook legs latchably extend into the corresponding holes in the daughter board and in the heat sink. Further more, an excess tolerance of the positions or of the dimensions of the clip may prohibit clip from being expectedly evenly located, thus resulting in insufficient and bad combination between the heat sink and the CPU that substantially affects the heat transfer efficiency of the CPU and the corresponding signal transmission. Additionally, the main body of the plate directly presses against the back surface of the daughter board, and thus may damage the circuits thereabouts.
Accordingly, it is known that another design is provided to use a locking pin with an associated surrounding coil spring wherein the distal end of the coil spring presses the bottom plate of the heat sink against the top surface of the CPU and the bottom end of the locking pin abuts against the back surface of the daughter board around the corresponding hole thereof through which the locking pin extends. Anyhow, the coil spring may provide, under some severe vibration situation, an unstable and unbalanced pressure on the heat sink that may jeopardize the securement between the heat sink and the daughter board, thus decreasing the heat transfer efficiency.
Accordingly, an object of the invention is to provide a fastening device including a locking pin and an associated pier-like clip, thus reliably pressing the heat sink against the CPU on the daughter board.